PARIS (AFP) - Links between the Nice truck attacker and "terrorist networks", including the Islamic State in Iraq and Syria (ISIS) militant group which has claimed the killings, "have not yet been established by the investigation", France's interior minister said Monday (July 18).

"We cannot exclude that an unbalanced and very violent individual" has been "through a rapid radicalisation, committed to this absolutely despicable crime," which killed 84 people, Bernard Cazeneuve told France's RTL radio.

Mohamed Lahouaiej-Bouhlel, a 31-year-old Tunisian, rammed a 19-tonne truck into a crowd leaving a fireworks display in the seaside city of Nice on Thursday night, killing 84 people and injuring about 300.

The ISIS group said one of its "soldiers" carried out the attack in response to its calls to target countries from the US-led coalition engaged in airstrikes against the group.

The attack was the third major terror strike on France in 18 months, and Cazeneuve hit out at opposition politicians who have fiercely criticised the government for not doing enough to stop the bloodshed.

Former president and main opposition leader Nicolas Sarkozy said Sunday that "everything that should have been done the past 18 months was not done".

Cazeneuve described the politicians as "shameful".

"Certain members of the political class have not respected the mourning period. Arguments broke out right away which personally saddens and shocks me," he said.